The Altec 421 isn't made for the deepest bass response. It makes a nice midbass/midrange driver though and would probably work reasonably well in a seven π. It just wouldn't generate much bass. That's really due to the driver's specs more than anything else.

Might try putting those BD15's in π cornerhorns instead. The BD15 woofer will work very nicely in the bass bin from a Professional or Audiophile series seven π loudspeaker, so one could stuff the bass cabinet with the BD15 woofer and put Oris horns on top instead of the π midhorn and tweeter.

The JBL 2226 sounds great in the seven π cornerhorn, as does the Eminence Magnum 15HO or Omega 15. I'm sure that the BD15 would sound excellent in that system too, response looks good and distortion must be low because of its neodymium magnet structure.